It wouldn't be a problem to implement: On each user's disconnect from server

Code:

time_played = ADDTIME(time_played, TIMEDIFF(NOW(), login_time))

that's very simple and fast, done in fraction of millisecond.

Calculating time_played for each player from the logs from past x years would reasonably need a separate computer and would probably last few hours - but it would only need to be done once. Performance is not a problem.

Now, if kids don't tell their parents to check /p, 99.9% of them will never come to the idea themselves so, neither we would have problems with privacy issues...